1. Field of the Invention
Exemplary embodiments of the present invention relate to an apparatus and method for power saving for a station; and, more particularly, to an apparatus and method for power saving for a station in a wireless communication system.
2. Description of Related Art
There are various standards introduced in a wireless communication market. A wireless communication network is classified into a wireless local area network (WLAN), a wireless personal area network (WPAN), and a wireless broadband access network according to a scope covered by one technology. Among them, a WLAN is one of most commercialized wireless communication technologies that support a short distance communication. In an early stage, a WLAN initially employed International standards IEEE 802.11b. In order to increase a data processing rate, the International standards IEEE 802.11b has been advanced to IEEE 802.11ac and IEEE 802.11ad through IEEE 802.11n. Lately, a WLAN employs IEEE 802.11ac and IEEE 802.11ad.
Due to the advance of a wireless communication technology, a wireless terminal supporting mobility has been introduced. One of major problems of a wireless terminal supporting mobility is a power problem. That is, a wireless terminal cannot continuously and stably receive power. A wireless terminal receiving a WLAN service has the same problem. In order to overcome the power problem, a wireless access network employing IEEE 802.11 provides various methods for saving power of wireless terminal. Particularly, such a power saving method is very important in a wireless communication service such as a Voice Over Internal Protocol (VoIP) service. In the VoIP service, users speak only 40% of a total talk time. That is, the users do not speak about 60% of the talk time. Accordingly, it is possible to save power using 60% of the talk time that the users do not speak. As a power saving method, a power save poll (PS-Poll) legacy power saving method was introduced in IEEE 802.11 and an unscheduled automatic power saving delivery (U-APSD) method was introduced in IEEE 802.11e. Many researches have proved that the U-APSD method is more suitable to a wireless communication than the PS-poll Legacy power saving method.
The PS-Poll based legacy power saving method and the trigger based U-APSD method have advantages and disadvantages as follows.
At first, the PS-Poll based legacy power saving method and the trigger based U-APSD method will be compared in a view of a frame collision rate of a terminal when an access point (AP) transmits buffered MSDUs through a downlink to a terminal. Here, MSDU stands for medium access control service data unit.
In the PS-Poll based legacy power saving method, all terminals transmit data after a beacon message including a traffic indication message (TIM). In trigger based U-APSD method, data is transmitted at a random moment. Therefore, the PS-Poll based legacy power saving method has a higher frame collision rate than that of the trigger based U-APSD method. A high frame collision rate means that a terminal is required to stay an awake state further longer during a back-off period. Accordingly, the PS-Poll based legacy power saving mode has low power saving effect in comparison with the trigger based U-APSD method in view of the frame collision rate.
Secondly, the PS-Poll based legacy power saving method is compared with the trigger based U-APSD method in a view of an additional signaling overhead which is caused signals generated by a terminal to receive MSDUs buffered in an access point (AP). In the PS-Poll based legacy power saving method, one PS-Poll is used to receive one buffered data. Accordingly, the PS-Poll based legacy power saving method has high signaling overhead. In the trigger based U-APSD method, a plurality of buffered MSDUs are received per one trigger frame according to a maximum service period (MAX SP) which is set up in a QoS information field. Accordingly, the trigger based U-APSD method has low signaling overhead in comparison with the PS-Poll based power saving method. High signaling overhead means that a terminal has to transmit more additional signals to transmit to an access point. Accordingly, a terminal has to stay in an awake state longer in the PS-Poll based legacy power saving method in comparison with the trigger based U-APSD method. Therefore, the PS-Poll based legacy power saving mode has worse power saving effect in comparison with the trigger based U-APSD method in a view of the signaling overhead.
Thirdly, the PS-Poll based legacy power saving method is compared with the trigger based U-APSD method in a view of a method of determining whether frames are buffered in an access point for downlink transmission. In the PS-Poll based legacy power saving method, a terminal determines whether or not a frame is buffered in an access point for downlink transmission by receiving and analyzing a traffic indication message (TIM) included in a beacon signal. In the trigger based U-APSD method, a terminal is required to additionally transmit a trigger frame for determining whether a frame is buffered in an access point for downlink transmission. Accordingly, the PS-Poll based legacy power saving method has better power saving effect in a view of the method of determining whether a frame is buffered in an AP for downlink transmission because the PS-Poll based legacy power saving mode is not required to transmit additional signal to determine the buffered frame in an access point.
As described above, the PS-Poll based legacy power saving method has better power saving effect in a view of the method for determining buffered downlink frames in an access point. However, the trigger based U-APSD method has better power saving effect in views of the frame collision rate and signaling overhead. Accordingly, it is necessary to develop a power saving method suitable in overall condition of a wireless communication environment.